But the Vigilant is not, repeat not, an unmodified civil G109b, it is a Vigilant T Mk.1, which has slightly different instruments, is flown with parachutes, has subtly different flying limitations, and (most importantly) an MTOW above the legal limit for a TMG.
G109b, civil CofA, MTOW=850kg
Vigilant T.1, Military CA Release, MTOW=908kg
JAR-22, the book the CAA certifies motorgliders against, says at paragraph 22.1(a)(2) "Single engined (spark or compression ignition) powered sailplanes, the design value W/bē (weight to spanē) of which is not greater than 3 (W[kg], b[m] and the maximum weight of which does not exceed 850kg"
W/bē still comes out at 2.98 (it's 2.8 at 850kg), but it's the MTOW that takes it outside the CIVIL definition.
[For what it's worth, I wrote the military CA release reports for the Vigilant. The CAA have also seen my logbook, with Vigilant hours (listed as such, don't call it a G109b) under light aircraft, and not queried it.]
If you want a comparison, there's a PFA homebuilt called a Minimax. Some variants are microlights, some are light aircraft - the difference is basically the fuel tank size and which particular engine is fitted. So you can have two nearly identical aeroplanes, one of which you log microlight hours on, and the other group A.
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